1. Field of the Invention
This invention relates to image pickup devices, and more particularly to image pickup devices in which electrical signals from an image pickup element are used in controlling the exposure of the element to an optical image.
2. Description of the Prior Art
To control such exposure, a method may be considered that, as the image pickup device is being exposed to an image of an object, luminance signals are integrated for one image frame, then the output of the integrator, in digital form, is computed with other various informations, and then the exposure is so adjusted that the result of the computation is brought into coincidence with a reference value. With only such means, however, the information representing the integrated value of luminance signals for one image frame immediately vanishes when resetting occurs in the vertical blanking period. The prior known device of this kind, because of its use of a sample-and-hold circuit for storing that information, has, therefore, a problem that the electrical circuitry becomes complicated.
Since particularly the analog-to-digital conversion takes a considerably long time to perform, it is of importance to hold the integrated value for at least that period of time.
Another problem of the prior known device is that the reference for integration, despite the change of the level of signals due to change of temperature of the image pickup element, has to be maintained constant, and, therefore, the luminance signal, or its integrated value, is caused to change although the brightness of the object remains unchanged. Thus, the use of this integrated information results in a bad influence on the accuracy of exposure control.